Environmental remediation processes are useful in a wide variety of industrial applications, including fuel, metallurgy, and glass processing applications, to name a few. Nitrogen oxides (NOx) can be present as a pollutant in a number of process emissions. For instance, combustion reactions can generate NOx emissions, such as reactions carried out by heaters, dryers, furnaces, and similar equipment.
Untreated NOx emissions can present numerous threats to humans, wildlife, plant life, and the environment. Exposure to NOx can cause irritation, adverse reactions, and sickness in humans and animals. Prolonged exposure can be fatal. NOx emissions (e.g., NO, NO2) released into the atmosphere can decompose on contact with water to produce nitrous acid (HNO2) and nitric acid (HNO3), e.g., acid rain, which can be highly destructive to plant life and man-made structures. NOx emissions (e.g., NO) can also react with oxygen in the lower atmosphere to form ozone, for example, by the reaction: NO+HC+O2+sunlight→NO2+O3. Ozone can cause health hazards to humans and wildlife, as well as causing damage to plant life. NOx emissions (e.g., NO2) can also contribute to smog, which can form when sunlight contacts a mixture of NO2 and uncombusted hydroarbons in the atmosphere.
Various environmental regulations, such as the Kyoto Protocol, have thus been put in place to reduce NOx emissions for the protection of society against harmful pollutants. Before these regulations were in effect, flue gases from combustion processes were vented directly into the atmosphere. As air quality regulations tighten and public awareness increases, however, industry leaders have begun employing various strategies for reducing NOx emissions. These strategies include, for example, pre-treatment, combustion modifications, process modifications, and post-treatment strategies for reducing NOx emitted into the environment.
Pre-treatment strategies include modifying or treating feed materials (e.g., fuel, oxidizer, and/or materials to be heated) to reduce the potential for NOx formation. Combustion modifications include changing the reaction process such as reducing excess air levels or air preheating. Process modifications include making changes to existing processes such as modifying equipment, firing rates, and/or thermal efficiency. Finally, post-treatment strategies can include removing NOx from exhaust streams after it has already formed. Reducing agents, such as CO, CH4 and other hydrocarbons, ammonia, etc., can be used to convert NOx into N2 gas. A catalyst can also be used to promote such reactions in some instances.
Post-treatment methods employing a catalyst are often referred to as selective catalytic reduction (SCR). SCR can control NOx emissions by reacting it with NH3 in a catalyst bed to form N2 and H2O. Conventional catalysts can include, for example, base metal catalysts, which can contain titanium and/or vanadium oxides and/or molybdenum, tungsten, or other elements. SCR can have numerous drawbacks including, for example, high material cost and/or process complexity, as well as the potential for catalyst plugging or poisoning by pollutants in the flue gases. Selective noncatalytic reduction (SNCR) is an alternative method which may provide various advantages in terms of process cost and/or complexity, as well as the ability to retrofit existing equipment. SNCR involves the reaction of a nitrogen-containing reagent, such as ammonia or urea, with NOx to produce nitrogen gas (N2), carbon dioxide (CO2), and water (H2O). For example, urea can be combined with water and used to treat NOx emissions, or ammonia can be added alone as a liquid or a gas, as shown by the following reactions.NH2CONH2+H2O→2NH3+CO2 4NO+4NH3+O2→4N2+6H2OThe reaction mechanism comprises the attachment of NH2 radicals to NO molecules and their subsequent decomposition into N2 and H2O.
While SNCR can be advantageous for the treatment of NOx emissions, these reactions also have a number of limitations. For example, SNCR reactions can be limited by an effective temperature range of about 870° C. to about 1100° C. At temperatures below about 870° C., the reagent and NOx may not react effectively. Unreacted reagent is generally undesirable because it can react with other combustion species to form undesired byproducts. For example, unreacted ammonia (often called “ammonia slip”) can react with combustion species such as sulfur trioxide (SO3) to form ammonium salts. At temperatures above about 1100° C., the reagent may decompose and form NOx rather than reduce it. For example, in the case of ammonia the following reaction can occur: 4NH3+5O2→4NO+6H2O. Accordingly, it can be important to maintain a sufficient residence time in the appropriate temperature window to maximize the efficiency of SNCR and minimize any potential downside. It would thus be advantageous to provide an efficient, cost-effective, easily operable process for treating exhaust streams comprising NOx. It would be further advantageous to provide SNCR methods with energy recycle and/or recovery features to efficiently heat exhaust gases to a temperature within the effective temperature range and/or to cool the exhaust gases before venting.